Abstract

The Holkerian (Lower Carboniferous, Mississippian) Hunts Bay Oolite (HBO) was studied in the south of Gower, along a section 15km long, up to 300m thick, at three locations: Langland Bay, Three Cliffs Bay and Port Eynon Bay. The HBO has been divided petrographically into four main associations (A, B, C and D), each with up to three lithofacies. Association A comprises crinoidal plates and bimodal grains of ooid, grapestone and aggregate grains it represents open-marine offshore deposits. Association B consists of the main oolite factory in an active shoal setting. Association C, deposited in back-shoal settings, and represents mainly grapestones and aggregate. Associations B and C are the main sandbody constituents. D is the lagoonal part of the HBO and includes bioclasts and peloids which are associated with sponges. The High Tor Limestone to HBO transition is gradational, without sharp boundaries. These associations accumulated in shoaling-upward sequences: offshore via active shoal to back shoal or lagoonal settings. Each sequence shows a thickening ESE to WNW trend. These cycles are not glacio-eustatic in origin but were likely tectonically-controlled and local, so that the sea-level curve of the HBO does not conform to the Holkerian global signal. Ramsay's (1987) and Scott's (1988) overall HBO-sandbody models are inapplicable to Ball's (1967) and Handford's (1988) marine sand-belt model as they lack differentiation between ramp and shield, and large bipolar cross-stratification. Considering all the HBO features exhibited, the deposystem lacks analogy to any modern or ancient examples, and represents a range of small sandbodies, and not major oolite shoals, as the source. A paragenetic sequence has been derived and fifteen diagenetic events identified. Most HBO diagenetic features differ from those of the well documented Chadian Gully Oolite of Gower, especially in the early diagenesis phase Gully Oolite contains ranges of emergence phases, and associated features such as vadose cementation and compaction, oomouldic fabrics, development of palaeosol and palaeokarsts.